Such a device is already known and generally comprises a single container in which the expansion chamber is separated from the water box of the radiator by a partition wall inside the container. This arrangement suffers from the drawback of not standing up very well to deformation under the effect of internal pressure variations, and the single container sometimes even cracks or breaks.
Preferred embodiments of the present invention provide a stronger water box and expansive chamber device which does not suffer from this drawback.